Shrinkage stoping

Shrinkage stoping is a mining method used for steeply dipping, narrower ore bodies with self supporting walls and ore. It is an overhand mining method that relies on broken ore being left in the stope to be used as the “working floor” and to support the walls. During the mining cycle, only 30 – 35% of the ore blasted is extracted being equivalent to the swell factor of in-situ ore to broken. When mining is complete to the next upper horizon, the ore is extracted. Although it is not necessary to fill the resulting voids, they are commonly filled with waste rock from development. Level intervals seldom exceed 40 meters due to uneven muck draw. Recoverable pillars are left at each level.

Optimum ore body characteristics for shrinkage stoping

Shrinkage stoping is used in situations where the body of ore has particular characteristics:

Dip

The minimum dip required is 50 degrees but with a dip this shallow, the ore will draw down from the hanging wall making for an uneven working surface. For efficient mining, a dip of 60 degrees or steeper is preferable.

Stability factors

The effectiveness of mining shrinkage methods depends on having strong wall rock and strong ore (back). There are no hard and fast rules to equate the rock quality versus span for shrinkage stoping. Because a span of over 3 meters will normally require ground support and also because muck size increases proportionately with the span, this method becomes more difficult at larger spans regardless of the rock quality. Rock quality should be at least RMR = 70.
Ground support in shrinkage stopes is problematic for the following reasons:

Support material has to be brought into the stope down a raise or up a steep manway.

Support material must be carried away from the entry point over broken muck and uneven surfaces.

Normally jacklegs and stopers are used for drilling making it difficult to automate ground support installation. Uneven working surfaces require a staging to be built or hazardous drilling conditions will exist.

Any ground support used in the back (roof) will end up in the muck after blasting. This in turn restricts even muck draw.

Ore content

Because the ore is captive for long periods in a moist environment, mining high sulphide content ore is not recommended because heating and possible spontaneous combustion can occur.

Length, width, regularity

Although there is no limit to the length that a shrinkage stope can be mined at narrow widths, stopes are normally designed so they can be mined from level to level in intervals that allow for the reserve ore to be removed. Widths exceeding 20 meters can be accommodated using multiple draw points from footwall to hangingwall or by leveling the muck with mechanical devices. Vein-type orebodies are most amenable to this mining method and fairly consistent widths are preferable. As mentioned earlier, larger spans may require ground support and the muck will inherently have a larger size distribution. Because the broken ore serves as the working floor, larger pieces tend to hang up, potentially causing a sudden and dangerous collapse of muck. This can also result in either the stope plugging from the inability to pull down the stope, or in overdraw resulting in the muck being too far from the back. If the width dimensions are erratic, mining this ore leads to creating benches or unstable walls. These benches will hold the muck and prevent an even draw down, creating a ledge where the ore will remain when the stope is pulled empty. It can also create a trap in which heavier minerals may be deposited and lost. This is particularly undesirable in high grade precious metal deposits.

Advantages of shrinkage stoping

The shrinkage method has several advantages:

Ore inventory

During development, all of the ore broken is recovered. Once mining starts, only the muck required to maintain a working level in the stope can be extracted. During mining, this is a disadvantage but once the stope is completed, the stope holds an inventory of ore that can be removed as needed. Normally, the grade of different draw points or chutes is known, so this inventory can also be used for grade control.

Costs

Given good ground conditions and the availability of reasonably paid, trained miners, shrinkage stoping can be cost effective. This method is not capital intensive. During the past decades, this mining method has become less common in North America because mechanization has made other methods cheaper and safer.

Ore production during development

If the orebody is developed by drifting on the vein, there is a steady supply of ore during drifting, creation of the draw system and driving of access raises.

Disadvantages of shrinkage stoping

Dilution

The most common method used to drill and blast in shrinkage stopes is to take horizontal breasts drilled with jacklegs. Dilution can occur if drilling is not well controlled and the walls are gouged at each blast. This also damages the walls, which in turn, may cause slabbing outside of the ore. Once a mine block is defined, the complete block must be mined and lower grade pockets must be extracted along with the rest of the ore. The method is not very selective and leaving waste pillars is difficult except at the extreme ends of the stope.

Safety

Shrinkage mining is one of the more hazardous methods. During development of the draw cone draw system, workers are continually exposed to open unsupported ground. Developing the draw cones using long holes alleviates some of this risk. Movement within the stope requires walking on broken muck that is uneven and at times unstable. The greatest hazard is the risk of a sudden movement of muck underfoot caused by the ore extraction. Severe injuries and fatalities have occurred when the ore suddenly dropped or when the crews below have mistakenly pulled the wrong chute or drawpoint. In most cases, access into the stope is down ladders from the level above. Access raises should be driven large enough to allow a slide to be built alongside the ladderway. This slide can then be equipped with a tugger hoist and supply bucket to reduce the manual ferrying of supplies on the ladders. However, once supplies are in the stope, they must be carried over the muck pile. Manways are normally only used as a second escapeway and for ventilation purposes.

Productivity

Levels of productivity are highly dependent on the width of the stopes and the amount of muck that can be extracted. Wider stopes allow for more tonnes to be broken per meter drilled but the resultant larger muck makes extraction more difficult. Expected productivity rates for stopes between 1.5 and 3 meters wide, with a two man crew drilling with one jackleg, are in the range of 20 to 60 tonnes per manshift. In wider stopes, where both miner drill, productivity will more than double. The ideal situation is for a two man crew to cycle a blast every shift – set up, drill and blast using jacklegs. Assuming an 8 hour shift, one driller can be expected to drill 50 to 100 meters, then load and blast. When stopes are less than 2 meters wide, drilling breast holes longer than 3 meters makes it difficult to keep the walls straight and the width allows for only one drill to be used. In wider stopes, the accuracy of the holes is not as important and therefore longer holes can be drilled.

Labour issues

Shrinkage mining requires skilled jackleg miners. These miners are required to use ladders on a routine basis and manually handle heavy and awkward loads. For these reasons, the popularity of shrinkage mining has decreased over the years in North America and elsewhere.

Unavailable ore inventory during mining

During the mining sequence, 65 to 70% of the ore must remain in the stope to maintain a working height from the muck surface to the stope back. This creates a large inventory of ore that must be carried until the stope is completed.

Stability factors

Shrinkage mining over a long strike length can result in pillars that are over- stressed in the later stages of mining. This situation can lead to squeezing in softer ground conditions or to rockbursting in stronger more brittle rock.

Shrinkage mining development

The most efficient method for the shrinkage mining development is to drive most of the openings in the ore. This reduces the amount of development and stopes can be put into production quickly. The disadvantage is that access past the stoping block is hindered or lost.

Lateral development

Ore is followed with a track or trackless drift with the dimensions determined by the equipment used. If access is required beyond the stope, a by-pass drift is driven on the footwall side approximately 10 meters away to allow drawpoints to be driven from this drift to the ore. With single access, the drift is driven along the ore and all development is done from this drift.

Drifts

Drifts advanced in ore should be driven at the determined dimension and not slashed out to the ore width. Ore delineation should make use of diamond drilling or test holes and not by slashing.

Extraction development

Ore level in the stope is controlled by pulling muck from the appropriate chute or drawpoint on the haulage level. These drawpoints should be approximately 10 meters apart centre to centre. Smaller spacings leave smaller pillars and larger spacings do not allow the stope to draw evenly.

Boxholes and chutes

Once the drift is advanced to the extents of the ore, vertical raises are driven and then angled to both sides along the ore to form a cone shaped pillar. The centre to centre optimal spacing for boxholes is 10 meters. The footwall of the boxhole raise is slashed at an angle to permit the installation of a chute. Some of these boxholes are made wider to permit a manway to be built alongside the chute.

Take down backs

A less common method is to take down the backs to approximately 5 meters and install a timbered back that includes chutes at 10 meter intervals. The advantage here is that more ore is recovered early and no pillars are left. This system is costly, labour intensive, and should only be used at the top of the ore body.

Drawpoints

Drawpoints can be driven either from the bypass drift or from the main ore drift. If mucking is trackless using scooptrams, the drawpoints are angled at a permissible turning radius for the equipment. Track muckers can also be used to load cars in the bypass drift and, in this case, the drawpoints are driven perpendicular to the ore.

From the end of the drawpoints, raises at approximately 60 degrees are driven to the ore, then the draw cones developed.

Raising

After lateral development is completed, a raise is driven from level to level at approximately 50 degrees. When nearing the level, the raise is turned to breakout on the side of the drift above so the breakthrough does not interfere with activity on the level. This raise is then equipped with a ladderway, a material handling slide, service pipes and cables.

Mining

Access

With all development complete, the stope will have a raise down from the level above and at least one manway up from the level below. In the early stages, workers may access the stope from below but supplies are easier to move down from above. A slide can be installed from below but this is awkward and does not allow for the construction of safety landings.

Ventilation

Ideally, the stope should be ventilated with natural ventilation flowing from the level below through the stope up the raise. The raise and manways should be placed so that the air flow supplies most of the stope. Auxiliary ventilation can be introduced as required.

Equipment

Jacklegs are normally used for production drilling and rock bolting the walls. Stopers are commonly used for rock bolting the backs.

Mucking the stopes depends on the system used. In track mines, with boxholes and chutes, only a train is required with cars that are filled from the chutes. Mucking with a train from drawpoints is done by side loading, using a track, or trackless overhead mucking machines.

Scooptrams mucking from drawpoints is the most efficient method to extract the ore. Scooptrams can load trucks or dump into orepasses. This method can be combined with track haulage by side loading rail cars.

When ore cannot be drawn evenly in a stope, slushers are used to scrape the muck to an area where the stope is drawing down. Although this is older technology, it is safer because the worker is not exposed to a sudden drop in muck levels and he can operate from a secure area.

Moving supplies into a stope is simplified by using a tugger winch attached to a flat bottomed bucket.

Material handling

Because the muck in the stope is always moving downward as ore is extracted, the working surface is uneven. In narrow vein stopes, it is preferable to pull the stope down ahead of the face so material and equipment can be kept where the ore is not likely to drop. The optimum configuration for safer material handling is to mine away from the raise used to supply the stope.

Drilling and blasting

Drilling is horizontal on a breast face and holes can vary from a length of 2.5 meters to 5 meters. Rock in shrinkage stopes is not normally difficult to drill and blast to provide good fragmentation. Since drilling is not an overly time consuming part of the daily cycle, sufficient holes should be drilled to ensure good fragmentation that in turn will allow for an even draw and a smoother working surface.

Ammonium nitrate and fuel oil (ANFO) type explosives are typically used along with portable compressed air loaders but cartridges are also used. Choice of detonator varies depending on availability, safety requirements and cost.

Muck extraction

Only 30 to 35% of the broken ore is extracted, depending on the swell factor of the in-situ rock. The miners, or supervisor, instruct the mucking crew as to what drawpoints to muck and how much to extract from each. Extreme caution must be employed by everyone involved because many accidents and incidents have occurred when instructions were not followed, wrong chutes/drawpoints were mucked, or muckers falsified reports.

The greatest hazard in shrinkage stopes is when the stope has advanced to an elevation where the muck does not move at the working floor of the stope as it is being drawn from below. Ordinarily ore in the stope drops noticeably when close to the mucking level. However, as the amount of stored ore increases, the muck tends to settle gradually, or not move, only to then suddenly drop. A person standing near the area can be pulled down and buried by this sudden release of hang-ups. If the wrong areas are drawn, this can also put the miners at risk. Good practice is to pull the muck down in advance of the face so that the force of the blast will both push the muck down and level the void. Known hang-up areas have to be isolated until the muck drops. Adding water from a safe distance to the affected section in the stope is the most common method of dislodging hang-ups from inside the stope while mucking takes place below.

Good communication between the mucking crew and the miners in the stope is essential. If blasting is necessary in the drawpoints/chutes during the shift, the miners must be alerted and consent to the blasting.

Post mining

Once the required sill level is reached, the equipment is removed and the back supported if necessary. Special care must be taken to remove all ladders and to barricade any entrance into the stope. The ore can then be pulled as required. Some grade control is possible if muck samples from the drawpoints were collected during the mining draw and sampling in the stope. In some gold mines where native gold is abundant, efforts are made to follow the muck down and wash the walls.
After all the available ore is removed, the drawpoints should be secured to eliminate further entry.

Pillar removal

Depending on the draw system used, a sill pillar will remain underfoot and the ore pillars forming the draw cones above. If a footwall haulage drift was used, the removal of the pillars can be accomplished using up and down longholes in a retreat sequence whereby level access is not lost. However, if all of the development was in the ore, removal of the pillars isolates the areas beyond the stope.

When the pillars are removed, the resulting ore is extracted from the levels below. If the pillars are highly stressed, care must be taken when drilling as it sometimes takes very little to trigger a rockburst.

Shrinkage stopes do not necessarily have to be backfilled unless the pillars will remain or there are stability issues that require backfill. When the stopes are remote from waste passes, they are sometimes used to store development waste.